It is increasingly recognised that integrated optical circuits have a number of advantages over electrical circuits. However, it has been difficult to produce integrated optical circuits which are comparably small, primarily due to the difficulty in producing waveguides which can include tight bends without large signal losses. It has also been difficult to produce integrated optical circuits including signal processing devices which can be easily coupled to current optical fibres, owing to a difference in the refractive index of the material used for optical fibres and those materials typically used for integrated optical devices, whilst still maintaining compact sizes.
Optical signals may be resonantly confined and manipulated using structures whose periodicity is of the same scale as an optical wavelength. Much recent interest has centred upon the field of photonic crystal (PC) waveguide structures.
Photonic elements may be incorporated in a range of different optical systems. Examples of appropriate optical systems include implementations in telecommunications, biosensors and optical storage media.
Photonic crystal waveguide structures are typically based on some perturbation in dielectric constant in the core of a planar waveguide structure. This has most commonly been performed by the spatially periodic etching of air rods through a cladding layer into the core layer of the waveguide. As light propagates through the core, it interacts with the dielectric constant modulation and, in some structures, in a manner analogous to electrons in a semiconductor, certain electromagnetic fields are forbidden to propagate in the core.
Electrons moving through a semiconductor lattice experience a periodic potential as they interact with the lattice nuclei via the Coulomb force. This interaction results in the formation of allowed and forbidden energy states. For pure and perfect semiconductors, no electrons will be found in an energy range called the forbidden energy gap or simply the band gap. However, the situation is different for real materials: electrons can have an energy within the band gap if the periodicity of the lattice is broken by, say, a missing silicon atom or by an impurity atom occupying a silicon site, or if the material contains interstitial impurities (additional atoms located at non-lattice sites).
Likewise, photons moving through a block of transparent dielectric material that contains a number of tiny air holes arranged in a lattice pattern also experience allowed and forbidden regions. The photons will pass through regions of high refractive index—the dielectric in the core layer—interspersed with regions of low refractive index—the air rods. This contrast in refractive index affects a photon just as a periodic potential would affect an electron travelling through a silicon crystal. Indeed, if there is large contrast in refractive index between the two regions then most of the light will be confined either within the dielectric material or the air rods. This confinement results in the formation of allowed energy regions separated by a forbidden region—the so-called photonic band gap
Furthermore, by not including certain holes/slots in the lattice/slot region, a defect state waveguide can be formed. More detail on the nature of the band structure of photonic crystals of this sort can be found in WO 98/53351 (BTG International).
One dimensional (1D) photonic crystals comprising a region of equidistant air slots formed in a core material, such that a photonic bandgap (PBG) is present, are known. Two-dimensional (2D) photonic crystals comprising a lattice of air holes formed in a core material, typically silicon, have been fabricated, which exhibit a photonic bandgap.
PCs are typically manufactured through a combination of PECVD or LPCVD (or ion sputtering), e-beam lithography or pattern masking, dry etching and oxidization processes. Conventionally the core may be made of silicon nitride (or Silicon Oxynitride or Ta2O5 while the buffer layer and cladding layer which bound the core are made of silicon dioxide (but also Silicon Oxynitride).
It is also known to introduce a third material into air slots/rods, in order to reduce out-of-plane losses. These are discussed in more detail in our co-pending applications U.S. Ser. No. 10/196,727 (filed Jul. 17, 2002) and U.S. Ser. No. 10/287,825 (filed Nov. 5, 2002)
Confinement of light within the waveguide is provided by using light having a wavelength within the photonic bandgap wavelength range. However, it has been found that photonic crystal devices suffer from large losses, mainly due to the escape of light from the waveguide in a vertical direction. Furthermore, in order to provide a strong and complete bandgap at optical frequencies, it has been necessary to use a high refractive index material, typically silicon. This makes it difficult to couple light into the waveguides from existing optical fibres, which typically have a core having a much lower refractive index. This problem necessitates complicated, lossy mode coupling devices.